Registering Custom Post Types and taxonomies with a theme? WordPress

REFUNDED

I have a using the Sahifia WodPress Theme from Themeforest.
I’m have created Custom Post Types and Taxonomies.
The theme has a frontpage creator that allows for adding WordPress category’s to create the frontpage.

The Image shows the themes category’s that I can add to the frontpage.

My issue is that I cannot add my custom post type to the frontpage builder as they are not showing.
Is it possible to ‘register’ my custom post type so that they register as a category and therefore available in the theme?

I’ve gone through various forums and can see references to registering the custom post types and taxonomies in functions.php.
Will this solve my problem and if so can someone please show me an example of how this is done?

There are one of two things that could be happening, either the theme is specifically targeting the 'categories' taxonomy when it looks for what to use for the front page builder, or two the post types and taxonomies you are creating aren't visible to the front page builder.

To check the second option you can use the add_post_type_support function http://codex.wordpress.org/Function_Reference/add_post_type_support to make sure your post type is using 'public' and 'show_ui' . Both those options are available when editing a post type in Types, but I'm not sure of the other. Try that to start

I checked with Types that 'public' and 'show_ui' and ticked and they are.
I then added the following to a file called custom_functions.php that the template uses and looks like it's called in from functions.php

If they are ticked inside Types then you shouldn't need the add_post_type function (for future use you don't need to add the init function around it, you just put the 1 line of code in your functions file).

I think Remy is right, you will need to have someone actually edit the theme hands-on. The best bet for doing that so that it doesn't get overridden might be to add a conditional filter that modifies the query to accept your custom argument, but its hard to tell without looking.

Thanks for getting back Kyle.
I tried adding the code above to the theme's functions.php, cutom-functins.php and theme-functions.php and nope.
My custom post types still do not register with the theme's homepage builder :-(